Presently available heat pumps, in the heating mode, reverse the function of the indoor and outdoor heat exchangers from that which we are accustomed to seeing on air conditioners. The indoor coil becomes the condenser and the outdoor coil (or heat exchanger) becomes the evaporator.
In a typical air-source heat pump (the most popular type), the refrigerant is metered to the outdoor evaporator coil such that the evaporating temperature is below that of the outdoor air. Heat energy flows therefore from the evaporator is compressed by the compressor and the hot discharge gas flows to the indoor coil which functions as the condenser. The condensing temperature being higher than the indoor air stream, allows heat flow to the indoor air.
As the temperature of the outdoor coil drops below freezing, it starts to frost up. This frost accumulation reduces that heat transfer rate of the coil surface and impedes the flow of the air through the coil. If the frost is not removed periodically, the heat pump will suffer a major loss of heating capacity.
Because of design temperature difference between the outdoor evaporator coil and the ambient, most air-source heat pumps start to frost when the outdoor ambient temperature falls below 45.degree. F., approximately.